A couple in Idaho claims to have new details in the mysterious case of missing Colorado mother Kelsey Berreth and a possible murder plot.

Patty and Joe Rockstahl claim they learned the information from one of their employees, a best friend of Krystal Lee, who allegedly had a relationship with Berreth’s fiancé, Patrick Frazee.

Frazee is charged in Berreth’s murder even though her body hasn’t been found. The Rockstahls told CBS News correspondent Nikki Battiste their employee told them that Lee and Frazee’s relationship began just after high school.

“She was in love with him from a very young age,” Patty Rockstahl said, adding, “She was very vulnerable and I think she cared for him, and he took advantage of that… He was her first love, I understand, and… he had a hold on her.”

A source tells CBS News the FBI questioned Lee regarding her possible role in Berreth’s disappearance and tells us she is cooperating.

The Rockstahls said they were told Lee, a nurse from Idaho and a mother, visited Frazee as recently as November – the month Berreth vanished. They claim their employee said Frazee threatened Lee, and she may have helped him dispose of evidence.

“Do you know what the threats were?” Battiste asked.

“Patrick told Krystal that little girls go missing off the playground all the time,” Patty said.

“Krystal was so scared, he was so angry that he said things that made Krystal think he had killed before. So of course, she’s going to be terrified,” Joe Rockstahl said.

The Rockstahls said they were told Frazee asked Lee three times as early as October to help kill Berreth, the mother of his child, but Lee refused.Asked what they heard was Frazee’s motive, Patty said, “That he felt that Kelsey was going to harm the child, that she was crazy.”

“Is Krystal Lee a murderer?” Battiste asked.

“I don’t believe so. I don’t,” Patty said.

Berreth was last seen on a surveillance video at a grocery store in Colorado on Thanksgiving Day with her and Frazee’s 1-year-old daughter.Three days later, police said Berreth’s cell phone pinged nearly 600 miles away in Gooding, Idaho. Several texts from the phone were sent including one to her employer saying she would be away for a week and another to Frazee.Patty said she thinks Lee sent the texts.

“She had the phone,” Patty said. “She was probably instructed to send those texts by Patrick but that’s – there’s nobody else that could’ve sent those texts to my knowledge.”

“Why would Krystal Lee, a nurse with children, help Patrick Frazee cover up the murder of his fiancée?” Battiste asked.

“I think she was afraid and I think she got in over her head,” Patty said. “She was in love with Patrick and we’ve all heard the stories of what people will do for love.”

The Rockstahls have never met Lee in person. She has not been charged with any crime related to Berreth’s disappearance. A jury trial for one of the two wrongful death suits Frazee is facing is scheduled to begin one month from today.